IMPACT OF WATER-STRESS ON LEAF ANATOMY AND ULTRASTRUCTURE IN CYNODON-DACTYLON (L) PERS UNDER NATURAL CONDITIONS

Seasonal changes in leaf anatomy and ultrastructure were studied in a sward of Cynodon dactylon (L.) Pers. grown under Mediterranean field c onditions and under water stress. Relative water content (RWC), leaf w ater potential (psi), and specific leaf weight were determined. Anatom ical measurements included leaf thickness and number and area of bundl e sheath and mesophyll cells. Quantitative measurements from electron micrographs were used to evaluate the subcellular structure of mesophy ll and bundle sheath cells from well-watered and water-stressed plants . In well-watered swards leaf mesophyll cell number was highest at the beginning of June, but in stressed plants the maximum was reached in the middle of July. Water stress decreased both mesophyll and bundle s heath cell areas during the experimental period, although there were n o significant differences in plant water relations in autumn. Ultrastr uctural changes in bundle sheath cells under stress included increase in starch deposition in chloroplasts, changes in the orientation of th ylakoids, and reduction in chloroplast area. Furthermore, water stress increased the cell wall thickness by 20%. Stressed mesophyll chloropl asts were characterized by an increase in the peripheral reticulum and in starch granules and a decrease in the amount of grana stacking rel ated to a decrease in leaf sodium concentration. The number of mitocho ndria per mesophyll cell was increased by water stress. Longer periods of stress induced folds in the outer tangential walls in bundle sheat h cells. In mesophyll cells alterations in cellular shape and in plasm alemma were observed, and the cytosol appeared to be markedly heteroge neous. In late summer the chloroplast envelope was swollen and distort ed; undulating dilated thylakoids were observed and the stroma appeare d less electron dense. In mitochondria, the matrix became progressivel y clearer. Environmental stresses influenced leaf anatomy and cell ult rastructure. Furthermore, ultrastructural changes could be related to variations in potassium and sodium contents.